When should I use multiscatter GGX?
What is it?

This is based on this paper.


  Modeling multiple scattering in microfacet theory is considered an
  important open problem because a non-negligible portion of the energy
  leaving rough surfaces is due to paths that bounce multiple times.


This effect can be clearly seen in high roughness glass, where a lot of light rays were killed because they happened to reflect on the glass itself rather than reflecting or refracting away, which you can see it the images provided by them:



Their model enable the sample (light) to bounce multiple times before being killed, Which result a more relastic and lighter result. It is also worth mentioning that this model enable more efficient energy conservation because the light will bounce till it loose all its energy.

This can be seen here, where the alpha represent the roughness of the diffuse and the glass shader:


However it doesn't really matter when the shader has low roughness since it is nearly imposible for the light to bounce on itself again, this can be seen in this shader with varity in roughness, you can see that the rough parts have a greater impact while smooth ones have almost no impact.



As for the speed of the rendering, using the multiscatter model will result only 2.5% slowdown which is pretty acceptable if you asked me.

When to use it?

Well it is pretty obvious that you should use it when you have shaders that have high roughness to avoid the dark areas. notice that the amount of darknening is directly proportional to the roughness of the shader, so if you have a semi low roughness you don't have to use multiscatter.